1. Field of the Invention
The present invention relates in general to a method of data transmission and reception, and more specifically, to a method of data transmission and reception using code-division-multiple-access (CDMA) technique.
2. Description of Prior Art
The spread-spectrum technique is a technique developed since about the mid-1950's. A detailed description of the conventional spread-spectrum systems can be found in a tutorial entitled "Theory of Spread Spectrum Communications--A Tutorial" authored by Raymond L. Pickholtz et al. and published on IEEE Trans. Commun., Vol. COM-30, pp. 855-884, May 1982.
The conventional BPSK (Binary-Phase-Shift-Keying) direct sequence spread spectrum communication system is shown in FIG. 1A-1B. A multiple access communication system that employs spread spectrum technique is technically termed as a code division multiple access (CDMA) system. The configuration of a basic CDMA system is shown in FIG. 2. A more detail description of the conventional BPSK-DS-SS (or BPSK-DS-CDMA) system of FIG. 1 will be given in the paragraphs under the header "Performance Evaluations."
The CDMA technique was developed mainly for capacity reasons. Ever since the analog cellular system started to face its capacity limitation in 1987, research efforts have been conducted on improving the capacity of digital cellular systems. In digital systems, there are three basic multiple access schemes: frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In theory, it does not matter whether the channel is divided into frequency bands, time slots, or codes; the capacities provided from these three multiple access schemes are the same. However, in cellular systems, we might find that one scheme may be better than the other.
A list of technical references pertinent to the subject matter of the present invention is given below:
[1] "Overview of Cellular CDMA", by William C. Y. Lee, IEEE Trans. Veh. Tech., Vol. 40, No. 2, pp. 291-302, May 1991.
[2] "On the Capacity of a Cellular CDMA System", by A. J. Viterbi, L. A. Weaver, and C. E. Wheatley III, IEEE Trans. Veh. Tech., Vol. 40, No. 2, pp. 303-312, May 1991.
[3]"A Statistical Analysis of On-off Patterns in 16 Conversations", by P. T. Brady, Bell Syst. Tech. J., Vol. 47, pp. 73-91, Jan. 1968.
[4]"Coherent Spread Spectrum Systems", by J. K. Holmes, John Wiley and Sons, New York, pp. 388-389, 1982.
[5] "Error Probability of Asynchronous Spread Spectrum Multiple Access Communications Systems", by K. Yao, IEEE Trans. Commum. Vol. COM-25, pp. 803-807, 1977.
[6] "Direct-Sequence Spread Spectrum Multiple-Access Communications with Random Signature Sequences: Large Deviations Analysis", by J. S. Sadowsky and R. K. Bahr, IEEE Trans. Inform. Theory Vol. 37, No. 3, pp. 514-527, May 1991.
[7] "Digital Communications and Spread Spectrum Systems", by R. E. Ziemer and R. L. Peterson, Macmillan, New York, Ch. 11, 1985.
[8] "Spread Spectrum Multiple Access Communications, Multi-User communication Systems", by M. B. Pursley, edited by G. Longo, Springer-Verlag, N.Y. pp. 139-199, 1981.
[9] "Performance Evaluation for Phase-Coded Spread-Spectrum Multiple-Access Communication-- Part II: Code Sequence Analysis", by M. B. Pursley and D. V. Sarwate, IEEE Trans. Commun., Vol. Com-25, No. 8, pp. 800-803, August 1977.
Remarkable results have been derived in the pertinent reference [2], "On the Capacity of a Cellular CDMA System" by A. J. Viterbi et al. This technical paper shows that the net improvement in the capacity of CDMA systems is four to six times better than that of a digital TDMA or FDMA system, and nearly 20 times better than that of current analog FM/FDMA system. Therefore, the CDMA scheme may become a major system in future communication systems.
The reason for the improvement in the multiple access capacity of the CDMA system mentioned above is that the capacity of the CDMA system is inversely proportional to cross-correlation noise, which is influenced or can be reduced by: (1) voice activity with a duty factor of approximately 3/8; and (2) spatial isolation through use of multi-beamed or multi-sectored antennas. Therefore if we can find another factor which can reduce the cross-correlation noise, the multiple access capacity will increase correspondingly.